1. Field of the Invention
The present invention generally relates to prime movers or heat pumps which operate over a closed thermodynamic cycle and, more particularly, to prime movers or heat pumps having a heat exchange fluid which is consistently in a liquid phase at all times during the thermodynamic cycle.
2. Description of the Prior Art
Many different machines which involve compression and expansion of a fluid (e.g. liquid or gas or a mixture of these phases) are known and used to change energy from one form to another to perform desired functions. Internal combustion engines and air-conditioning systems are particularly well-known and familiar examples of such machines. (As used hereinafter, the term "machine" will be used to refer generically to a device operating as either a heat pump or a prime mover. Theoretically, any thermodynamic system can potentially be operated as either a prime mover or a heat pump depending on whether heat is added to or rejected by the system.) Many of these types of devices, such as air conditioning systems, operate in a closed cycle in which the fluid is recirculated.
Many different thermodynamic mechanisms which can be employed in such machines are well-known and may be exploited with greater or lesser efficiency, depending on the design of the machine. Certain theoretical thermodynamic cycles having distinct properties are known by the names of their principal investigators such as the Stirling cycle which is characterized (for a heat pump) by constant volume heat rejection and constant temperature compression and expansion. As another example, the Brayton cycle (again for a heat pump) is characterized by constant pressure heat rejection and constant entropy expansion and compression. In both of these cycles and other known theoretical cycles, certain parameters are kept substantially constant during certain portions of the cycle and energy is often constrained to be ideally removed from or added to the system by variation of a single other parameter. Also, for a prime mover rather than a heat pump, in any theoretical ideal thermodynamic cycle, heat would be input rather than rejected.
Vapor compression machines operating in a closed cycle, such as most air-conditioners and refrigerators, operate by condensation and evaporation of a fluid since the change between phases is accompanied by a very large change of energy and volume of the material. The temperatures at which such condensation and evaporation can be carried out, however, is largely dependent on the properties of the fluid and the conditions under which it is contained. For this reason, so-called chlorofluorocarbons (CFCs) have become popular for use in air-conditioning and other heat pump applications (e.g. where mechanical energy is used to effect heat exchange) because of the temperatures at which heat exchange must take place. However, in recent years, extremely serious environmental damage has been attributed to release of chlorofluorocarbons into the atmosphere from such heat exchange systems (and other sources) and alternatives yielding similar efficiencies and convenience with environmentally neutral materials are being actively sought. Water remains one of the major materials of choice for prime movers (e.g. where energy is applied to the system as heat and removed as mechanical energy, as in a steam engine or turbine) but efficiency remains a serious concern for closed systems where the water must be condensed and recirculated.
While no viable alternatives have existed, it should be noted that the high compressibility of gaseous phase materials have required compressors and expanders of substantial volume in systems exploiting phase change of the heat exchange fluid. Therefore, in large-scale air-conditioning installations, for example, substantial space must be dedicated to the compressors. Heat exchangers also occupy substantial space because of the amount of heat which must be absorbed or rejected during evaporation and condensation. Since this space has an economic value, it must be considered as a cost of operating such systems. Reduction in the size of heat exchangers must often be accompanied by the capacity for increasing the differential of temperatures at which heat exchange takes place; increasing the capacity of compressors and the pressures at which they operate and thus the amount of energy input thereto with consequent decrease of system efficiency. This trade-off between energy input requirements and system size has made highly efficient heat pump installations very difficult and expensive when all economic costs are considered, especially for shipboard applications.
While ideal liquids have been traditionally regarded as incompressible and thermodynamically inert, about seventy years ago, it was noted by John Malone that some liquids may be compressed and expanded with substantial efficiency of conversion of heat to mechanical energy under conditions of temperature and pressure near the critical point of the liquid. For this reason, any ideal regenerative thermodynamic system employing all-liquid (e.g. consistently liquid during all portions of a thermodynamic cycle) heat exchange fluid is commonly referred to by the name "Malone" as a prefix to the name by which the ideal system is known. Several engines employing all liquid phase heat exchange fluid are reported to have been built by Malone and tested, following a Stirling cycle implemented with reciprocating pistons. While fairly high efficiencies relative to prime movers of that period were reported, the engine was not sufficiently advantageous to support commercialization at that time. Malone-type systems continue to be a subject of sporadic investigation but no way to exploit a Malone-type cycle with a sufficient degree of the efficiency theoretically available therefrom has heretofore been found to make a practical implementation of such a cycle in a machine competitive with other commercially available machines for performing desired functions. A summary of the state of the art in Malone-type cycles and an overview of the theoretical operation thereof for refrigeration is given in "Malone Refrigeration" by Greg W. Swift published in ASHRAE Journal, November, 1990, pp. 28-34. This article discusses a test heat pump constructed by the author and operating on a Stirling cycle using propylene but indicates that the design was principally concerned with versatility for quantitative characterizations of loss mechanisms and without concern for efficiency, size, cost or reliability. The article also suggests that pressurized carbon dioxide may be a suitable working fluid and that a Malone-Brayton cycle heat pump could be used for refrigeration.
The use of an expander mechanism is well-known in prime movers, such as jet engines, to provide an expander mechanism to extract mechanical power from the system, usually for driving the compressor and other ancillary equipment, such as generators. It is also known in some all gas phase Brayton cycle heat pump applications to provide an expander mechanism to extract mechanical power and reduce the amount of mechanical input power required. As with jet engines, it is common, for mechanical simplicity, to operate the expander and compressor on the same shaft. Since the functions of the expander and compressor are generally considered to be complementary functions and, as indicated above, liquids have classically been considered to be of substantially constant density for purposes of thermodynamic analysis (e.g. ideal liquids being regarded as incompressible), it has been the common practice to arrange for the fluid handling capacities (e.g. displacement, volume per revolution, etc.) of the expander and compressor to be the same, including previous demonstrations of Malone-type cycle machines.